GALA on JUICE Part 3 – The challenge of radiation exposure on the 'Mount Everest of the Solar System'
The Ganymede Laser Altimeter (GALA) will face one of the most hostile environments in the Solar System while in the Jupiter system. The space around the planet is saturated with an enormously high level of radiation, so strong that it can degrade the performance of orbiting scientific instruments or even destroy them. GALA is one of ten instruments on board the JUICE mission, which will set off for the fifth planet of the Solar System in April 2023. It was meticulously developed and extensively tested to survive and function correctly in this extreme environment.##markend##
Jupiter is the 'Mount Everest of the Solar System' in terms of radiation levels. The Solar System's fifth and most massive planet has the strongest planetary magnetic field. It extends over several million kilometres and captures charged particles such as protons, electrons and ions from the solar wind and the volcanic ejecta of the moon Io. The magnetic field accelerates these particles, turning them into small, charged projectiles that will constantly bombard GALA. Unlike most other planets, Jupiter's 'doughnut-shaped' radiation belt is largely dominated by electrons, which have a higher penetration depth than protons and ions – requiring more shielding material to protect the inner, sensitive parts of the instrument.
In our daily lives, we may see examples of radiation shielding such as the lead coat used in X-ray scans or the thick metal sphere around nuclear reactors. Unfortunately, the interaction of electrons with such shielding produces bremsstrahlung, namely gamma rays, which are high-energy photons with an even greater penetration depth than electrons and can reach even the most well-shielded parts of the instrument. During the closest approach to Jupiter, the radiation flux experienced by GALA will be more than half a million times higher than the average value on Earth – corresponding to more than 7700 mammograms per day.
The most radiation-sensitive parts of GALA are the detector and the electronic components in the laser and on the circuit boards. The laser rods are the components in which the laser light is generated, and they are particularly sensitive to radiation, so special laser crystals were developed and qualified for GALA. They are called neodymium-doped yttrium aluminium garnet (Nd:YAG) crystals and are not doped with chromium (Cr3+) ions as, although chromium reduces radiation sensitivity, it also considerably reduces the laser's power.
It was therefore decided to use pure laser crystals but to protect them with strong shielding. Optical elements such as lenses or mirrors are also very sensitive to radiation: high doses of radiation can cloud or darken their surfaces. The use of electrically insulating materials such as plastics and polymers was strictly prohibited as the electric charge accumulated by the electron bombardment can cause unwanted electric arcs – as in the case of a balloon rubbed against a piece of clothing – which physically destroy the material.
The telescope's carefully selected surface coatings absorb stray light and reflect the laser signal and their optical properties may be altered by ionisation. It is not possible to shield external surfaces exposed to space, so the selected materials must be inherently radiation resistant. As a result, the telescope parts most exposed to space radiation will be irradiated with several gigarads over the course of the entire mission. This value is almost exclusively reached on Earth within nuclear reactors.
Our main focus during the development of GALA was protecting the electronics, especially the sensitive detector and laser components. The entire device was designed according to this concept: the sensor and the laser were placed in the centre, and everything else was shaped to create a kind of cylinder (ideally a sphere) around them.
This design required unorthodox solutions: the analogue electronics board (see images below), for example, had to be shaped like a hexagon with a hole in the middle because the detector unit had to be plugged into it.
We sourced very robust electronic components qualified for the expected radiation levels. We often travelled to facilities such as the ELBE centre in Dresden, the GEODUR facility at ONERA in Toulouse or the University of Delft to irradiate custom parts and new technologies with electrons and qualify them for the extreme conditions at Jupiter.
However, the selection of radiation-resistant parts and the testing were not enough on their own: additional shielding was still required. For this reason, the GALA housing was built with particularly thick walls. Usually, scientific instruments that will fly to space are designed to be as light as possible. With GALA and all instruments on JUICE, weight had to be traded for radiation hardness. The design of the instrument was revised several times following detailed radiation analyses. The complex interaction between the charged particles and all the components of GALA was simulated using special software. Each stage of development in the design required hundreds – sometimes thousands – of hours of computing time to predict the radiation doses accumulated by all sensitive parts over the course of the mission. These analyses have confirmed that GALA is now capable of surviving and functioning correctly despite the harsh radiation environment at Jupiter.